Add support for transactions spanning multiple actions

[aschrijver]

Provide a mechanism for transaction boundaries to span multiple actions.

See this post on the Vert.x google group: https://groups.google.com/d/msg/vertx/eIjzdQI23ss/hjYUtJcxVKcJ

[aschrijver]

Could use a similar mechanism as implemented by Tim Yates in the mod-jdbc-persistor:
https://github.com/timyates/mod-jdbc-persistor/blob/master/src/main/java/com/bloidonia/vertx/mods/JdbcProcessor.java#L385

[aschrijver]

Both OrientDB and Neo4J start implicit transactions when you create a Graph through GraphFactory (i.e. TransactionalGraph), but these are bound to a single transaction.

I tried the mechanism implemented in mod-jdbc-persistor but found that it doesn't work. In OrientDB nothing is persisted when a different thread performed the commit, and in Neo4J an exception is thrown.
Changing the verticle settings in mod.json to single-thread, non-worker solves the issue on OrientDB. However the persistor must be a multithreaded, worker verticle

Tinkerpop has a ThreadedTransactionalGraph for multi-threaded access to the graph, but there are no implementations for OrientDB and Neo4J.
See: https://github.com/tinkerpop/blueprints/wiki/Graph-Transactions#threadedtransactionalgraph-and-multi-threads-for-one-transaction

Also in Tinkerpop3 there will be significant changes to the way that transactions are handled. See: https://groups.google.com/forum/#!topic/gremlin-users/7ydt3Vkzsrs